2. Field of the Invention
The present invention relates to a process for continuously producing dimethyl carbonate. More particularly, the present invention relates to an industrial process for continuously producing dimethyl carbonate by a catalytic reaction in the gas phase of carbon monoxide with methyl nitrite in the presence of a solid catalyst, while collecting dimethyl carbonate by absorbing it in an absorbing medium, and effectively recovering and re-using non-reacted methyl nitrite accompanying the dimethyl carbonate absorbed in the absorbing medium.
Dimethyl carbonate is a compound useful as a material for synthesizing aromatic polycarbonates, medicines and agricultural chemicals, and as a solvent.
2. Description of the Related Art
A conventional industrial process for producing dimethyl carbonate by a catalytic reaction in the gas phase of carbon monoxide with methyl nitrite in the presence of a solid catalyst comprises, as disclosed in U.S. Pat. No. 5,214,185, a first step of catalytically reacting carbon monoxide with methyl nitrite in the gas phase in the presence of a solid catalyst in a reactor, the step of absorbing the resultant dimethyl carbonate of the first step by an absorbing medium comprising dimethyl oxalate in a dimethyl carbonate-absorbing column, the third step of bringing a non-condensed gas fraction prepared in the second step into contact with molecular oxygen and methyl alcohol in a methyl nitrite-regenerating column, to regenerate methyl nitrite from nitrogen monoxide contained in the non-condensed gas fraction, and a fourth step of distilling-collecting dimethyl carbonate from the dimethyl carbonate-containing absorbing medium in an extract-distilling column and a dimethyl carbonate-distilling column.
In the above-mentioned process, methyl nitrite is substantially not consumed in the total of the reactions as shown in the following reaction formulae:
CO+2CH.sub.3 ONO.fwdarw.CO(OCH.sub.3).sub.2 +2NO PA1 2CH.sub.3 OH+2NO+1/2O.sub.2 .fwdarw.2CH.sub.3 ONO+H.sub.2 O, PA1 a first step of introducing a feed gas containing carbon monoxide and methyl nitrite into a reactor and catalytically reacting carbon monoxide with methyl nitrite in the gas phase in the presence of a solid catalyst in the reactor, to prepare a reaction product gas comprising dimethyl carbonate mixed with a by-product containing nitrogen monoxide; PA1 a second step of bringing the reaction product gas into contact with an absorption medium comprising dimethyl oxalate in a dimethyl carbonate-absorbing column, to provide a liquid fraction containing dimethyl carbonate absorbed by the absorbing medium and a non-condensed gas fraction containing nitrogen monoxide and non-reacted carbon monoxide and methyl nitrite; PA1 a third step of bringing the non-condensed gas fraction of the second step into contact with molecular oxygen and methyl alcohol in a methyl nitrite-regenerating column, to regenerate methyl nitrite from the nitrogen monoxide and provide a regenerated gas fraction containing the regenerated methyl nitrite; and PA1 a fourth step of collecting dimethyl carbonate from the liquid fraction of the second step, PA1 wherein, before the fourth step, the second step liquid fraction is brought into contact with carbon monoxide, to recover methyl nitrite accompanying the second step liquid fraction and to allow the recovered methyl nitrite and the carbon monoxide to be incorporated into the non-condensed gas fraction of the second step in the absorbing column; and after the third step, at least major portion of the resultant regenerated gas fraction of the third step containing carbon monoxide and methyl nitrite is recycled as a feed gas to the reactor of the first step.
and thus it appears as a substantial catalyst. However, the loss of a portion of methyl nitrite fed to the dimethyl carbonate-producing process is inevitable due to the absorption of methyl nitrite by the absorbing medium, and partial discharge of the circulating gas during the first, second and third steps. Therefore, a certain amount of methyl nitrite or nitrogen oxides must be added to the feed gas. To reduce the supplemental feed of methyl nitrite, it is important to recover non-reacted methyl nitrite with high efficiency during the process.